Magnetic trip units, which are used in electric circuit breakers and comprise a yoke and a hinged armature, are known from the prior art. Trip units are used to monitor a current flowing in an electric line and to trip a circuit breaker in the event of a so-called overcurrent, said circuit breaker interrupting the current flow in the monitored line. An overcurrent here is a current with a current strength which is so far above the rated current strength of the monitored line and the electrical devices connected thereto that there is a risk of damage to the devices or the line itself due to the heat produced. A trip unit is referred to as a magnetic trip unit, if the circuit breaker is tripped by the trip unit using magnetic forces.
One class of electric circuit breakers is what are known as compact power circuit breakers, which can typically switch currents of 10 A to approx. 2500 A and fault currents up to many times the rated current. Power circuit breakers are typically accommodated in an enclosing housing. The individual current phases can be switched in what are known as pole cartridges. To this end a pole cartridge comprises a housing, in which a moving contact and a fixed contact are accommodated, which can be mechanically separated and brought together to switch the currents on and off.
When the moving and fixed contacts of a pole cartridge are separated, an arc is produced, which is extinguished in an extinction chamber. The arc ionizes the gas in the extinction chamber and produces an overpressure in the extinction chamber, which is equivalent to the arc energy. Power circuit breakers are also known, which do not contain pole cartridges but do contain moving and fixed contacts which are otherwise shielded in their enclosing housing.
Power circuit breakers typically use magnetic trip units for instant short circuit tripping. To this end the current path passes through a magnet system so that a magnetic field action and thus a force action can be achieved on a magnetic or hinged armature. The current-carrying system is passed through an arrangement of magnetic bracket (yoke) and hinged armature. The hinged armature here is supported on the side of the yoke so that it can flip closed due to the magnetic forces.
To adjust the magnetic trip unit a defined force has to be applied by the yoke to the hinged armature in all positions of the magnetic trip unit. It is therefore necessary to keep the friction and support of the hinged armature constant over the life of the power circuit breaker.